Gear ratio calculations?

Ok, heres everything I know.

Motor is a 3HP 2-Stroke, probably revs up to 10,000 rpm, since its very small. So it would probably be happy around 7000RPM

I would like to shoot for 20MPH, I want more torque than speed, I want to be able to bomb around in the yard.

My Rear wheel is 26"

Now, ive get everything that everyone has said, but the _one_thing_ I am unsure of is how to get speed. Is it revs/wheel circumfrence?

This is what I understand 60/8 = 7.5 so 7.5:1, 8000RPM/7.5 = 1,066 rpm at the crank fo the bike. Right. Then...

Just for fun, i Took 1066RPM and diveded it by 26" for the wheel, and go 41, so would that be 41MPH? I doubt it, im almost sure im wrong

TJ Poseno scribed in :

congratulations TJ, you're worse at math than I am (-:

swarf, steam and wind

-- David Forsyth -:- the email address is real /"\

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Thanks, now I know how ot do it...

At least you post there helped me.

First off: When you say "at the crank of the bike", are you intending to input the power to the bike via the pedal crank? (yes, possible. No, not likely to be super easy) If so, you're also going to need to know the ratios of the gearing (if any) the bike has due to the sprockets and chain. Same process of math: This chainwheel has X teeth, the chain is sitting on this other chainwheel, which has Y teeth. The ratio of this gearset is Z. From there, you multiply Z by whatever the total ratio is from your gear-train off the motor to get your "final drive ratio" to give you total effective reduction (or overdrive, if your bike's gearing can result in an overdrive situation)

Second: You say you've got a 26 inch wheel. Especially in the case of bikes, wheels are usually referred to by their diameter, not their circumference, so going with conventional use, I'm needing to assume that's what you meant. Anything else leaves me going "Where the heck did he find a 4.something inch diameter (26 / (2 * pi) = 4.14 and change) bicycle wheel????"

Unless that's a bad assumption on my part, the circumference isn't 26 inches. It's 2 * pi * r, or in this specific case, 2 * pi * 13 (r is the radius - which is half of the diameter. pi, of course, is good old 22/7 or 3.14 and change) so your equation is 2 * 3.14 * 13 = 81.64 inches. Which means the bike moves forward 81.64 inches per revolution of the wheel.

How many RPMs from the motor? Let's go with your 8000 from above.

Let's also use the 7.5:1 gear ratio that's been established.

8000 / 7.5 = 1066.6 RPM at the output.

Let's further assume that you are, as you appear to be indicating, putting that 1066.66 RPM into the crank of the bike, and that the gears of the bike are set to yield a ratio of 1:1 (just to keep the math simpler - the actual ratio is probably going to be significantly different, and may indeed be variable in the case of a multi-gear mountain bike or similar. If that's the case, then you'd need to multiply the 7.5 final output from the motor/gear-train by whatever the curently selected ratio on the bike is to give you the "at the axle" RPM of the wheel) A 1:1 will just pass the revs straight through unchanged, leaving you with 1066.66 RPM at the wheel. Which means that the bike is moving 1066.66 * 81.64 = 87082.122 inches per minute. 87082.122 * 60 (minutes in an hour) means it will travel 5224927.3 inches per hour if the input speed is kept constant for that long.

Let's see... a mile is 5280 feet, multiply by 12 (inches in a foot) to find out there are 63360 inches to a mile.

5224927.3/63360 then equals 82.46 miles per hour.

May I suggest you get a *REALLY REALLY* good helmet (on second thought, forget the helmet... go for full body armor) and a large life insurance policy before you go out and start "bombing around the yard"? Feel free to name me as beneficiary on the life insurance policy. :)

7000 RPM sounds a bit high for a 3 hp twostroke. Most outboard motors are redlined at about 5500 RPM.

At 5000 RPM with a 26" wheel, a ratio of 12.8 : 1 will get you 30 mph, a ratio of 19.3 : 1 gets you 20 mph.

Wheel RPM = 12.93 * speed (mph)

The old Whizzer motorbikes used single stage reduction with a belt: The wheel has a large pulley on it, not much smaller than the wheel diameter. If you could contrive a 22" dia pulley on your 26" wheel, the drive pulley on the motor might be 1.2 to 1.8" dia. You'd probably need an idler pulley to make the belt wrap around the drive pulley as much as possible. That could also serve as a clutch: reduce pressure on idler to relieve belt tension so it can slip.

Had one of those many years ago.

Formed of sheet metal and fastened to all the spokes to hold its shape.

The Whizzer motor pulley was a lot bigger than that. The belt ran in a groove on the outside of the centrifugal clutch. It has been a long time but I feel that the drive pulley was 4 or 5" diameter. The engine was a four stroke and, IIRC, didn't turn up any 5 grand. It could do 30 or 35mph and needed some help from the pedals on significant hills. Also, it used a coaster brake so the pedals weren't flinging around when you weren't pushing on them. They served as foot pegs when not needed for aux power.

A guy I knew at school had one of the English bikes that had a 2-stroke motor mounted above the back wheel. It had a ridged metal wheel that was pressed against the rear tire.

Ted

When you calculate anything, you really want to include the units. Taking 1066 rpm and dividing by 26 inches, you may get 41 for the numeric part, but the units are revolutions / minute inch. And that makes no sense.

If you take 26 inch diameter and multply by Pi ( circumference per diameter ), you get circumference/dia times dia or just circumference. or about 82 inches. So for one revolution the bike moves 82 inches. Then 1066 rpm times

82 inches / revolution equals 106682 inches per minute. If you divide that by 12 inches per foot, you get 8890 feet per minute. And if you divide by 5280 feet per mile, you get 1.68 miles per minute. Next multiply by 60 minutes per hour and you get 101 miles per hour. So if you want about 20 mph, you need to divide the rpm by 5.

Dan

Maybe you made the common usenet mistake of getting your calculator and Ouija bourd mixed up? One of the nice things about usenet is that usually someone will pick up on mistakes, as nearly everybody makes mistakes.